Dislocations represent one of the most fascinating and fundamental
concepts in materials science(1-3). Most importantly, dislocations are
the main carriers of plastic deformation in crystalline materials(4-6).
Furthermore, they can strongly affect the local electronic and optical
properties of semiconductors and ionic crystals(7,8). In materials with
small dimensions, they experience extensive image forces, which attract
them to the surface to release strain energy(9). However, in layered
crystals such as graphite, dislocation movement is mainly restricted to
the basal plane. Thus, the dislocations cannot escape, enabling their
confinement in crystals as thin as only two monolayers. To explore the
nature of dislocations under such extreme boundary conditions, the
material of choice is bilayer graphene, the thinnest possible quasi-two-
dimensional crystal in which such linear defects can be confined.
Homogeneous and robust graphene membranes derived from high-quality
epitaxial graphene on silicon carbide(10) provide an ideal platform for
their investigation. Here we report the direct observation of basal-
plane dislocations in freestanding bilayer graphene using transmission
electron microscopy and their detailed investigation by diffraction
contrast analysis and atomistic simulations. Our investigation reveals
two striking size effects. First, the absence of stacking-fault energy,
a unique property of bilayer graphene, leads to a characteristic
dislocation pattern that corresponds to an alternating AB AC change
of the stacking order. Second, our experiments in combination with
atomistic simulations reveal a pronounced buckling of the bilayer
graphene membrane that results directly from accommodation of strain. In
fact, the buckling changes the strain state of the bilayer graphene and
is of key importance for its electronic properties(11-14). Our findings
will contribute to the understanding of dislocations and of their role
in the structural, mechanical and electronic properties of bilayer and
few-layer graphene.